From 91ebcf22af627772635cb28edbe6bd19ad22bc07 Mon Sep 17 00:00:00 2001
From: Zakary Timson <zaktimson@gmail.com>
Date: Mon, 11 Dec 2023 21:50:40 +0000
Subject: [PATCH] Add regex.md

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+# R* Cheatsheet
+
+## Modifiers
+
+| Symbol | Description                   |
+|--------|-------------------------------|
+| `g`    | Global - search entire string |
+| `i`    | Case-insensitive              |
+| `m`    | Multiple lines                |
+| `s`    | Treat as single line          |
+| `x`    | Allow comments & whitespace   |
+| `e`    | Evaluate placement            |
+| `U`    | Un-greedy patterns by default |
+
+## Escape
+
+| Symbol | Description               |
+|--------|---------------------------|
+| `?#`   | Comment                   |
+| `\ `   | Escape special characters |
+| `\Q`   | Begin literal sequence    |
+| `\E`   | End literal sequence      |
+
+## Anchors
+
+| Symbol | Description       |
+|--------|-------------------|
+| `^`    | Start of line     |
+| `$`    | End of line       |
+| `\A`   | Start of string   |
+| `\Z`   | End of string     |
+| `\b`   | Word boundary     |
+| `\B`   | Not word boundary |
+| `\<`   | Start of word     |
+| `\>`   | End of word       |
+
+## Characters
+
+| Symbol | POSIX                                | Description                              |
+|--------|--------------------------------------|------------------------------------------|
+| `\c`   | `[:cntrl:]`                          | Control character                        |
+| `\S`   |                                      | Not a white space                        |
+| `\s`   | `[:blank:]` `[:space:]`              | White space                              |
+|        | `[:punct:]`                          | Punctuation                              |
+| `.`    | `[:print:]` `[:graph:]`              | Printed Character, with & without spaces |
+| `\W`   |                                      | Not a letter                             |
+| `\w`   | `[:alpha:]` `[:upper:]`  `[:lower:]` | Letter                                   |
+| `\D`   |                                      | Not a number                             |
+| `\d`   | `[:digit:]`                          | Number                                   |
+| `\x`   | `[:xdigit:]`                         | Hexadecimal digit                        |
+| `\O`   |                                      | Octal digit                              |
+| `\n`   |                                      | New line                                 |
+| `\r`   |                                      | Carriage return                          |
+| `\t`   |                                      | Tab                                      |
+| `\v`   |                                      | Vertical tab                             |
+| `\f`   |                                      | Form feed                                |
+| `\xxx` |                                      | Octal number "xxx"                       |
+| `\xaa` |                                      | Hexidecimal "aa"                         |
+
+## Repeat
+
+| Symbol   | Range       | Description                           |
+|----------|-------------|---------------------------------------|
+|          | `{x}`       | Exactly `x` times                     |
+|          | `{min,max}` | Repeat between `min` & `max` times    |
+| `?`      | `{,1}`      | 0 or 1                                |
+| `*` `*?` | `{0,}`      | 0 or many (Adding `?` make un-greedy) |
+| `+` `+?` | `{1,}`      | 1 or more (Adding `?` make un-greedy) |
+
+Un-greedy repeaters will take the minimum number of characters required
+
+## Group Selectors
+
+| Symbol         | Description                                |
+|----------------|--------------------------------------------|
+| `.`            | Wildcard/Single character (Except newline) |
+| `(abc\|123)`   | `abc` or `123`                             |
+| `(...)`        | Capture group                              |
+| `(?<name>...)` | Named capture group                        |
+| `(?:...)`      | Group un-captured                          |
+| `[abc]`        | `A` or `B` or `C`                          |
+| `[^abc]`       | Not `A` or `B` or `C`                      |
+| `[0-9]`        | Number between `0` and `9`                 |
+| `[a-g]`        | Lowercase letter between `a` and `g`       |
+| `[A-G]`        | Uppercase letter between `A` and `G`       |
+
+All selectors are inclusive
+
+## Reference Group
+
+| Symbol  | Description    |
+|---------|----------------|
+| `$n`    | N'th Group     |
+| `$\` ` | Before matched |
+| `$'`    | After matched  |
+| `$+`    | Last match     |
+| `$&`    | Entire match   |
+
+Some regex implementations use \ instead of $.
+
+## Assertions
+
+| Symbol       | Description             |
+|--------------|-------------------------|
+| `?=`         | Lookahead               |
+| `?!`         | Negative lookahead      |
+| `?<=`        | Lookbehind              |
+| `?!=` `?<!`  | Negative lookbehind     |
+| `?>`         | Once-only subexpression |
+| `?(...)`     | If condition            |
+| `?(...) \| ` | If-else condition       |
+